Protective device



Dec. 20, 1927.

B. E. SHA CKELFORD ET AL PROTECTIVE DEVICE Filed Feb. 15. 1921 INVENTORSBENJH M/N E. SHHCKELFOED WILL/HM H- 7: HOLDEN.

ATTORNEY Patented Dec. 20, 1927.

UNITED STATES v 1,653,375 PATENT OFFICE.

BENJAMI N ESTILL SHACKELFORD, OF EAST ORANGE, AND WILLIAM HENRY TOWNEHOLDEN, OF BLOOMIIELD, NEW JERSEY, ASSIGNORS TO WESTINGHOUSE' LAM?COMPANY, A CORPORATION OF PENNSYLVANIA.

PROTECTIVE DEVICE.

Application filed February 15, 1921. Serial No. 445,247.

Our invention relates to protective devices and more particularly todevices for protecting electrical circuits from high-potentialdischarges or high-tenslon sources likely to prove dangerous to life ordestructive to property. I

, More specifically our invention relates to protective devices of thetype 1n whlch refractory electrodes are enclosed in an envelopecontaining a rarifiedgas. D

An object of our invention is the provlsion of a protective devicehaving greater current-carrying capacity than those heretofore in use.

Another object of one form of our invention is to provide means forautomatically grounding the circuit when the gaseous pressure in thedevice exceeds a predetermined value, thus affording protection againstvoltages in excess of those desired to be passed over a line, even whenthe gaseous pressure in the device is too high to pass a discharge.

Although inert gases have heretofore been used in protective devices ofthe rarifiedgas type to reduce the resistance of the electrode gap, suchgases have proved unsatisfactory in performing this function for thereason that the current discharge from the circuit to be protected bydevices of this type is small. y

We have discovered that, if the electrode gap contains a mixture ofcertain gaseous substances, at least one of which is a gas of lowdielectric cohesion, such, for instance, as mercury vapor, the currentdischarge is comparatively large, whereby a greater degree of safety isafforded to the circuit to be protected.

A convenient way of furnishing the metallic vapor'to the electrode gapis to provide a pool of mercury, either within the envelope or in a sidetube communicating therewith; the communication of the side tube withthe envelope being preferably retained, although, in some cases, it maybe desirable to seal or tip off the side tube in the manner usuallyemployed in tipping off an incandescent lamp.

In the preferred arrangement in which the source of mercury vapor is incommunication with the electrode enclosure, a pool furnishes a supplyfrom which the vapor of mercury may be evolved.

A mixture of certain gases and vapors has peculiarities that adapt itespecially to use in a protective device of the type hereincontemplated. It has been found that a mixture of a metallic vapor and amonatomic gas is particularly suitable. The amount of metallic vaporactually present before the discharge begins is small, owing to the lowvapor pressure of even volatile metals at three millimeters of mercury,the addition of a drop of mercury raises the pressurerequired to give asparking potential of 400 volts to about 20 millimeters, while, at thesame time, the current in the discharge at any given voltage increasesseveral hundred percent. The resulting device has the advantage ofdischarging considerable current when the applied voltage is but littlein ex' cess of the sparking potential.

It will also be seen that the mixture of metallic vapor and gas hasanother advantage over a single vapor or gas in that the sparkingpotential is more definite and less subject to variation, for the reasonthat the gaseous impurities which may be liberated from the walls of thedevice by the passage of electric discharges, constitute a smallerpercentage of the total pressure of the mixture than of the lowerpressure which must be employed when either a gas or a vapor is usedsingly. It will thus be seen that the presence of the metallic vaporgreatly increases the rate of productionof-ions and also lowers thepotential required to start ionization by collision and cause a currentto flow through the mixture of gas and metallic vapor.

We have further discovered that, if the source of the metallic vapor,which may be a pool of mercury, is spaced from the electrodes, theformation of a mercury arc is prevented. This is advantageous because,should an arc form, the potential drop across the terminals of-theprotective device might fall to a value below the normal line voltage ofthe circuit to be protected, resulting in the persistence of thedischarge of the increase in pressure, it is desirable to provide meansfor short-circuiting the device.

Iir order that the invention may be more fully understood reference ishad to the accompanying drawing in which the single figure is a sideview of one form of device embodying the invention.

In this View an envelope 9 is provided with a plurality of preferablyparallel depending line electrodes 10 and 11 included in, or connectedto, the circuit'to be protected, and a grounded electrode 12, preferablylocated between the line electrodes. The intermediate portions of thepair of line electrodes diverge at 13 and the lower'ends converge at 14,in the manner illustrated, the grounded electrode 12 preferably being astraight wire. The envelope 9 has connected to its lower end one branchof a depending U-shaped fluid-filled tube 15 which communicates with theenvelope and has its unconnected end closed.

It. is desirable to employ aliquid substance, since the float elementswhich form part of this modification require a liquid medium for theiroperation. The liquid should, however, be of such character as tofurnish a vapor which will decrease the resistance of the electrode gapsufiiciently to cause the circuit to be protected, as hereinbeforeindicated. Mercury fulfills these requirements, since it is liquid andhas a comparatively high vapor pressure at ordinary temperatures. Hence,we prefer to fill the U-tube with mercury. It is evident that othersubstances or mixtures might be used instead of the mercury, as abuoyant medium, although some such substances or mixtures might evenrequire the application of heat to create a sufficient vapor pressure togive satisfactory service. A float 16, comprising a buoyant part 17, iscarried on the mercury column in the tube 15. The part 17 has anLip-standing staff 18,

preferably consisting of wire and carrying a shorting ring 19 at itsupper end adapted to short the line electrodes.

While we have shown the short circuiting element in the form of a ring,it is to be understood that any other form, such as a disc, forinstance, which bridges the line electrodes, or the line electrodes andthe grounded electrode, will suflice. The level of mercury and thelength of the staff 18 are adjusted so that the shorting ring 19 takes aposition within the diverging and converging parts to permit the bud 'ngelement to remain free from, or clear 0 the electrodes when in normalposition, and to come into contact with either the diverging or converging portions when moved upwardly or downwardly, respectively, fromthis position. The mercury level in the discharge-tube side will eitherfall when the increase in pressure is on the envelope side of the deviceor rise when the increase in pressure is on the closed branch side ofthe U-shaped tube.

In order to ensure the vertical position of the staff 18, its upper endmay be conveniently guided on the grounding electrode, in the mannershown in the drawing, in which the shorting ring surrounds the groundedelectrode. Other means may, of course, be employed to guide the staff,for example, a guide eyelet might be secured to the upper end of thestaff and have sutlicient diameter to slide freely on the groundingelectrode; or the staff might be guided in other ways and from otherpoints, particularly in case the grounding electrode should be dispensedwith.

Manifestly, instead of the short-circuiting ring being located withinthe space bounded by the diverging and converging parts of theelectrodes, the short-circuiting ring might surround the electrodes anddrop down on the diverging parts when the mercury in the U-shaped tubeshould change its position.

What is claimed is:

1. In a protective device, the combination of a plurality of electrodes,an enclosure for said electrodes having a column of mercury the heightof which is regulated by the pressure within said envelope, a floatcompris ing an electro-conductive portion moi'able with the mercurycolumn and adapted to short circuit said electrodes when the pressurewithin said envelope reaches a predetermined point.

2. An electric discharge device comprising an envelope containinggaseous filling, a plurality of electrodes in said envelope, a body ofliquid contained therein, a float on said liquid and means opcrativelyassociated with said float for short circuiting said electrodes when thepressure within the device reaches a predetermined point. 7

In testimony whereof. we have hereunto subscribed our names this 14thday of Feb ruary, 1921.

BENJAMIN ESTILL SHACKELFORD. WILLIAM HENRY TOWNE HOLDEN.

